Abstract: A mathematical-physical model is established to study the influence of charge parameters on the flow field characteristics for 12.7 mm underwater machine gun. Fluent software, combined with user- defined function and dynamic grid technology, is used to simulate the flow fields around the sealed muzzles of underwater machine guns with the charge quality of 15.5 g, 13.0 g and 11.0 g, respectively. The results show that the axial movements of initial air and projectile are impeded by the high-density water, resulting in the slowdown of projectile and the gathering of gunpowder gas at muzzle during the projectile flying through the muzzle. The attenuation of gas jet pressure follows the exponential decay rule after a projectile is fired from the muzzle. When the jet is affected by both the projectile velocity and the gas jet pressure, its shape gradually changes from a trapezoidal cavity to a gourd-like cavity, and the maximum displacement of jet follows the exponential decay rule. The initial formation time of Mach disk is basically the same for the three charge conditions. With the reduction in the initial velocity of projectile and the initial injection pressure of gas, the gunpowder gas gathers in the jet head and radially expands to form the secondary jet. Meanwhile, the impact time of projectile base on the shape of Mach disk is shortened, helping the shock core faster approximate to normal shock wave. It can be seen that the influence of charge quality on the distribution of the flow field around the muzzle has a certain regularity. Key

Key words: machinegun, underwatersealedlaunch, muzzleflowfield, gasjet, numericalsimulation